This invention concerns a process for the oxidation of sulfonamides to sulfonyl imines using chromium (IV) dioxide as oxidant.
There are no known routes to sulfonyl imines of the structure R1R2Cxe2x95x90NSO2R3 via oxidation of the corresponding sulfonamides.
S. M. Weinreb, Topics in Current Chemistry, Volume 190, 131-184 broadly reviews the preparation and chemical reactions of N-sulfonyl imines.
F. A. Davis et al., J. Org. Chem., 1997,62, 2555-2563 disclose processes for the preparation of enantiomerically pure sulfinimines.
A Viso et al., J. Org. Chem., 1997, 62, 2316-2317 discuss the cycloaddition reactions of sulfinimines.
T. Fukuyama et al., Tetrahedron Letters, Vol. 36, No. 36, pp. 6373-6374 report the preparation of 2- and 4-nitrobenzenesulfonamides and their use as amine protective groups with easy chemical removal by thiolates.
R. A. Lee and D. S. Donald, Tetrahedron Letters, vol. 38, No. 22, pp. 3857-3860, 1997 disclose the utility of Magtrieve(copyright) chromium dioxide as an oxidant for the conversion of alcohols to aldehydes and ketones.
U.S. Pat. No. 5,698,744 discloses the utility of chromium dioxide for a variety of oxidation processes. Oxidation of alcohols and activated hydrocarbons are exemplified.
There is a need for a process for the oxidation of sulfonamides to corresponding sulfonyl imines.
This invention discloses the use of chromium dioxide as a suitable oxidizing agent for the preparation of sulfonyl imines. The use of chromium (IV) dioxide provides the advantage of using magnetism to separate residual chromium from the desired product.
Also disclosed is a process for the oxidation of sulfonamide of the structure I 
where R1 and R2 are independently selected from the group consisting of hydrogen and organic radicals not possessing a hydrogen atom in the position beta to the nitrogen of the sulfonamide group; and R3 is an organic radical selected from the group consisting of alkyl, substituted alkyl, aryl, and substituted aryl groups, and optionally, R1 and R2 can form a ring, to sulfonyl imines of the structure II, 
wherein R1, R2, and R3 are as described above, said process comprising:
(a) optionally, contacting a sulfonamide of the structure I with a suitable solvent or liquid;
(b) contacting the sulfonamide of the structure I with chromium (IV) dioxide under oxidizing conditions whereby a sulfonyl imine product is produced; and
(c) optionally, isolating the sulfonyl imine product.
Another disclosure of the present invention is a process for the preparation and in-situ use of imines of the structure III,
R1R2Cxe2x95x90NHxe2x80x83xe2x80x83III
from corresponding sulfonyl imines of the structure II 
having iminyl functionality and wherein R1 and R2 are independently selected from the group consisting of hydrogen and organic radicals not possessing a hydrogen atom in the position beta to the nitrogen of the sulfonamide group, and wherein R3 is an organic radical selected from the group consisting of alkyl, substituted alkyl, aryl, and substituted aryl groups, and optionally, R1 and R2 can form a ring, comprising:
(a) contacting the sulfonyl imine of the structure II with a reagent that is capable of reacting with the iminyl functionality of the structure II to form a derivative of structure III having a sulfonyl group; and
(b) removing the sulfonyl group.
A further disclosure of the present invention is a process for the preparation and in-situ use of a combinatorial library of imines of the structure III,
R1R2Cxe2x95x90NHxe2x80x83xe2x80x83III
from corresponding sulfonyl imines of the structure II 
having iminyl functionality and wherein R1 and R2 are independently selected from the group consisting of hydrogen and organic radicals not possessing a hydrogen atom in the position beta to the nitrogen of the sulfonamide group, and wherein R3 is an organic radical selected from the group consisting of alkyl, substituted alkyl, aryl, and substituted aryl groups, and optionally, R1 and R2 can form a ring, comprising:
(a) contacting the sulfonyl imines of the structure II with one or more reagents that are capable of reacting with the iminyl functionality of the structure II to form at least one derivative of structure III having a sulfonyl group; and
(b) removing the sulfonyl group.
This invention relates to a process for the oxidation of sulfonamides to produce sulfonyl imines. The invention lies in the use of chromium (IV) dioxide (CrO2) to afford yields which are generally greater than 50% and purities greater than 70% under reaction conditions that are relatively easy to carry out.
One source of chromium (IV) dioxide is E. I. du Pont de Nemours and Company, Wilmington, Del., which sells the material under the trade name Magtrieve(copyright). Magtrieve(copyright) is a recyclable magnetic chromium (IV) dioxide.
In the process of the present invention, 
the sulfonamides of the structure I and resulting sulfonyl imine products of the structure II may bear a wide variety of organic substituents. R1 and R2 are selected from the group consisting of hydrogen and organic radicals not possessing a hydrogen atom in the position beta to (i.e., 2 carbon atoms away from) the nitrogen of the sulfonamide group. R1 and R2 may be hydrogen, alkyl, alkenyl, substituted alkyl or alkenyl, alkynyl, and aryl, including phenyl, substituted aryl and heteroaryl, and optionally, R1 and R2 can together form a ring. Preferably, R1 and R2 are selected from the group consisting of hydrogen, optionally, substituted aryl and optionally, substituted heterocyclic groups. Specifically exemplified R1 and R2 substituents are hydrogen, phenyl, p-fluorophenyl, p-methoxyphenyl, 2-furanyl, and 2-thiophenyl.
R3 is an organic radical selected from the group consisting of alkyl, substituted alkyl, aryl, and substituted aryl groups. Specifically exemplified R3 substituents include 2-nitrophenyl, 2,4-dinitrophenyl, p-tolyl, p-methoxyphenyl, methyl, and trifluoromethyl.
The sulfonamide of the structure I may be prepared by contacting an amine of the structure R1R2CHNH2 wherein R1 and R2 are independently selected from the group consisting of hydrogen and organic radicals not possessing a hydrogen atom in the position beta to the nitrogen, with a sulfonyl chloride of the structure R3SO2Cl. Other methods known in the art may be used also.
Sulfonyl imines may be prepared by the process disclosed at temperatures from about 20xc2x0 C. to about 200xc2x0 C., preferably from about 50xc2x0 C. to about 200xc2x0 C., and most preferably from about 100xc2x0 C. to about 130xc2x0 C.
The process is most conveniently carried out at atmospheric pressure. Super- or sub-atmospheric pressure may be used if desired.
The process can be carried out with the sulfonamide at least partially dissolved in a suitable solvent or liquid. A wide variety of solvents or liquids may be used. Suitable solvents and liquids are aprotic, non-nucleophilic liquids that are unreactive to the chromium dioxide oxidation reagent under reaction conditions. Preferred solvents and liquids are aromatic hydrocarbons (for example, benzene or toluene), substituted benzenes (for example, chlorobenzene or dichlorobenzene), halogenated aliphatic hydrocarbons (for example, carbon tetrachloride and dichloroethane), acetonitrile, nitromethane, esters, and ethers.
The reaction mixture is heterogeneous. Agitation by means customary in the art is acceptable.
Upon conclusion of the reaction, particulate chromium dioxide is conveniently removed from the liquid organic substrate/solvent phase by filtration, centrifugation or magnetic separation.
The process of this invention is useful in the synthesis of organic compounds (specifically sulfonyl imines). These compounds have utility as intermediates in the synthesis of further compounds and in combinatorial chemistry techniques.
The present invention also discloses a process for the in situ use of N-unsubstituted imines of the structure III,
R1R2Cxe2x95x90NHxe2x80x83xe2x80x83III
wherein R1 and R2 are independently selected from the group consisting of hydrogen and organic radicals not possessing a hydrogen atom in the position beta to the nitrogen of the sulfonamide group. N-unsubstituted imines often are unstable and cannot be isolated for use in further reactions. The process and in situ use is defined to mean generating, in effect, an N-unsubstituted imine that is protected by a sulfonyl group. This protected imine is used in the desired reaction. Optionally, the product is then deprotected by the removal of the sulfonyl group to produce the desired final product.
The process comprises (a) contacting the sulfonyl imines of the structure II with one or more reagents that are capable of reacting with the iminyl functionality of the structure II to form at least one derivative of structure III having a sulfonyl group and (b) removing the sulfonyl group.
A suitable reaction is one in which an N-unsubstituted imine will react with the reagent to form the desired product wherein the sulfonyl group is inert to all other reagents and solvents used under the reaction conditions. The optional deprotection step can be any performed method known in the art. By xe2x80x9cdeprotectionxe2x80x9d is meant the step in which the sulfonyl group is removed.
To illustrate, one specific example is a [4+2] cycloaddition reaction where R3 is a nitro-substituted benzene substituent. Addition of ammonia to aldehydes and ketones does not form stable N-unsubstituted imine adducts. Trimers or other polymers are formed instead. In this example, a nitro- or dinitrobenzenesulfonyl imine serves as the synthetic equivalent of an N-unsubstituted imine. After subsequent reactions, nitro- and dinitro- sulfonamide groups, which are stable to acids and bases, are efficiently removed using nucleophilic reagents, for example thiols and primary amines, including polymer supported nucleophiles (e.g. polymer supported thiols and amines).
An added feature of the sulfonyl imine protecting group in this example is the electron withdrawing property of the aromatic ring. The methine CH of a nitro- or dinitro-benzenesulfonyl imine is significantly more electrophilic and more reactive than the methine CH of other imines.
The above-described process of the cycloaddition/deprotection is illustrated below. 
Since the process described above is facile, performed under mild conditions requiring no catalyst, and results in purities generally over 70%, it may be used to generate and utilize a combinatorial library of N-unsubstituted imines by using one or more of the sulfonamides of the structure I to produce one or more sulfonyl imine products of structure II.
The process described is useful as an equivalent to obtain the desired product. The following reaction equation would be impossible to perform due to the instability of N-unsubstituted imine. 
Another example of processes of the instant invention is detailed below, wherein R1, R2, R3, R4, and R5 are hydrogen, alkyl, alkenyl, substituted alkyl or alkenyl, alkynyl, and aryl, including phenyl, substituted aryl, and heteroaryl, and optionally, R1 and R2 can form a ring, and wherein Nuc indicates a nucleophilic group Such as, but not limited to, a thiol, a primary or secondary amine, a Grignard reagent, a phosphine, and a halogen. 
This reaction also lends itself well to combinatorial processes and the creation of a combinatorial library.
Additional reactions in which the sulfonyl imine of structure II can be substituted for the imine of structure III that can be used in the processes of the instant invention include, but are not limited to, nucleophilic additions, [2+2] cycloadditions, and ene reactions to form xcex2-amino acids, xcex1-aminonitriles, amines, cyclic amines, and xcex2-lactams among others illustrated below and described in Weinreb, ibid. 
All of these reactions can he adapted to combinatorial processes and the creation of a combinatorial library as well.
As used herein, a combinatorial library is an intentionally created collection of a plurality of differing molecules which can be prepared by selected synthetic means and screened for the desired activity or characteristic in a variety of formats (e.g., libraries of soluble molecules, libraries of compounds attached to resin beads, silica chips, or other solid supports). The libraries are generally prepared such that the compounds are approximately in equimolar quantities, and are prepared by combinatorial syntheses. Combinatorial syntheses refers to the parallel syntheses of diverse compounds by sequential additions of multiple choices of reagents which leads to the generation of large chemical libraries containing related molecules having molecular diversity. Screening methods for libraries vary greatly and are dependent upon desired activity, size of library, and class of compounds in the library.
The libraries of the instant invention can be of any type. These types include, but are not limited to, arrays and mixtures. Arrays are libraries in which the individual compounds are simultaneously synthesized in spatially segregated locations, typically identified by their location on a grid. Mixture libraries contain a mixture of compounds that are simultaneously synthesized and assayed. Identification of the most active compound is then performed by any of several techniques well known in the combinatorial art (for example, deconvolution).